Molecular+Shape

The VSEPR model is used to determine molecular shape. It assumes arrangements that minimize repulsion of electron pairs around the central atom. The presence of four bonding pairs produces a tetrahedral arrangement. Three such pairs produce s trigonal planar shape, unless there also is a lone pair on the central atom, in which case the shape is trigonal pyramidal. Two bonding pairs produce a linear shape, but if there also are two lone pairs on the central atom, the shape will be bent.

During bonding, atomic orbitals can undergo hydbridization, or mixing to form new, identical hybrid orbitals. An s orbital and three p orbitals can hybridize to form four identical sp3 hybrid orbitals. That produces a tetrahedral, trigonal pyramidal, or bent molecular shape, depending on whether none, one, or two of these orbitals contain lone pairs (as in the case of CH4, NH3, and H2O, respectively). The hybridization of an s and two p orbitals produces three sp2 hybrid orbitals, which produces a trigonal planar shape (as in the case of AlCl3). The hybridization of an s and one p orbital produces two sp hybrid orbitals, which produce a linear shape (as in the case of BeCl2).